The invention relates to an ultrasound diagnostic apparatus capable of imaging an inspection object such as an organ in a living body by transmitting and receiving ultrasonic beams so as to generate an ultrasound image used for examination or diagnosis of the inspection object, an ultrasound image generation method, and a recording medium recording a program; and particularly relates to an ultrasound diagnostic apparatus, an ultrasound image generation method, and a recording medium recording a program capable of setting an optimum sound velocity searching range based on a value of a sound velocity calculated previously, reducing sound velocity calculating time, and thus improving a frame rate.
Conventionally, ultrasound examination apparatuses such as ultrasound image diagnostic apparatuses using ultrasound images are put to practical use in the medical field. Generally, an ultrasound examination apparatus of this type has an ultrasound probe including therein a plurality of elements (ultrasound transducers), and an apparatus body connected with the ultrasound probe. Ultrasonic beams are transmitted toward an inspection object (subject) from a plurality of elements of the ultrasound probe, and ultrasonic echoes from the subject are received by the ultrasound probe. Then, the received ultrasonic echo signals are electrically processed in the apparatus body so as to generate an ultrasound image.
When an ultrasound image is generated in the ultrasound examination apparatus, ultrasonic beams are transmitted with a focus on a region to be examined of a subject, e.g., an organ in a living body, a lesion in the organ, or the like from the elements of the probe, and ultrasonic echoes from a reflector body of the region to be examined, e.g., a surface or an interface of the organ, the lesion, or the like are received through the plurality of elements. However, a plurality of elements receive ultrasonic echoes reflected by the same reflector body, and thus as compared with ultrasonic echo signals reflected by the reflector body positioned at a focus position of ultrasonic beams transmitted from a transmission element and received by the transmission element, ultrasonic echo signals reflected by the same reflector body and received by elements other than the transmission element are delayed. Because of this, the ultrasound examination apparatus analog-to-digital (A/D) converts ultrasonic echo signals received by a plurality of elements into element data, and then performs reception focusing processing on element data. That is, the ultrasound examination apparatus performs delay correction to match phases and performs phasing addition to generate sound ray signals, thus generating an ultrasound image based on the obtained sound ray signals.
In conventional ultrasound diagnostic apparatuses, the sound velocity of ultrasonic waves in the subject is assumed to be constant, and reception focusing processing was performed by fixing a sound velocity of the ultrasonic waves to a certain value set in advance.
However, the sound velocity varies depending on the type of tissues such as fatty layers or muscular layers in a living body, and therefore the sound velocity of ultrasonic waves is not uniform in the subject. In addition, the thicknesses of fatty layers or muscular layers are different between an overweight subject and a slim subject. In other words, the sound velocity of ultrasonic waves varies from person to person.
Accordingly, in a conventional ultrasound diagnostic apparatus in which the sound velocity of ultrasonic waves is fixed, when the actual sound velocity in a subject differs from a set sound velocity, the arrival time for the ultrasonic echoes to be reflected inside the subject and reach the elements does not match with a set delay time.
As a result, there is a problem in that proper phase matching is not possible, reception focusing is not properly performed, and the image quality of the obtained ultrasound image becomes inferior. In addition, there is also a problem in that the obtained ultrasound image is distorted with respect to the actual subject.
With respect to such problems, in the ultrasound diagnostic apparatus, the sound velocity in the subject is determined (calculated), and reception focusing processing is performed using this sound velocity.
For example, JP 2011-92686 A describes an ultrasound diagnostic apparatus which transmits and receives ultrasonic waves after setting a region-of-interest where a diagnosis region (in an ultrasound image) is divided, calculates a focus index for each of a plurality of sound velocities (set sound velocities) set in advance in each region-of-interest by performing reception focusing processing with respect to obtained element data using a plurality of sound velocities (set sound velocities) set as appropriate, and uses the calculated focus indexes to determine the sound velocity in the region-of-interest.
Examples of the focus indexes include contrast value, brightness, and the like. For example, a set sound velocity where the brightness set as the focus index was the highest may be determined as the sound velocity in the region-of-interest.
JP 2011-92686 A describes that the delay time correction, the phasing addition, and reception focusing processing is performed based on the sound velocity determined using focus indexes, and then envelope detection processing is performed after correction for attenuation so as to form a brightness image (B mode image).